1. Field of the Invention
The present invention relates to an optical element such as a retardation plate, a polarizing plate, and a color filter for a display. In particular, the invention relates to an optical element comprising a liquid crystal Layer, made from a liquid crystalline composition including a polymerizable liquid crystal material, as a refractive index anisotropic layer, and a method for manufacturing the same. In this specification, the term “liquid crystal layer” is used for the meaning of a layer optically having the property of a liquid crystal, and those having a solid phase state solidified while maintaining the molecular arrangement of the liquid crystal phase are also included as the layer state.
2. Description of the Related Art
In general, the liquid crystal is used as a display medium such as a display element, represented by the TN (twisted nematic) type and the STN (super twisted nematic) type, utilizing the reversible motility of the molecular arrangement. Also, the liquid crystal is used as an optical element such as a retardation plate, a polarizing plate and a color filter for a display, utilizing the alignment property and the refractive index anisotropy.
Here, as to the applications of the latter, recently, a large number of optical elements comprising a liquid crystal layer made from a polymerizable liquid crystal material, as a refractive index anisotropic layer, have been proposed. Specifically, for example, Japanese Patent Application National Publication (Laid-Open) No. 2002-533742 discloses an optical element having the function of the wavelength selective reflecting property and the polarization selective reflecting property, which is produced by using a special polymerizable liquid crystal compound. Moreover, Japanese Patent Application Laid Open (JP-A) No. 5-215921 discloses a birefringence plate produced by using a polymerizable liquid crystal compound having a rod like structure. Furthermore, JP-A Nos. 8-338913 and 9-152509 disclose an optical compensating sheet produced by using a polymerizable liquid crystal compound having a disc like structure.
A generally used optical element has a supporting member, such as a plastic film, and a liquid crystal layer (refractive index anisotropic layer) made from a polymerizable liquid crystal material laminated on the supporting member via an alignment film.
Here, the alignment film provided in between the supporting member and the refractive index anisotropic layer has the alignment limiting force of limiting the alignment direction of the liquid crystal molecules in the refractive index anisotropic layer. Such an alignment film can be formed by, for example, forming a polymer layer of a polyimide, a polyvinyl alcohol, gelatin, etc., having the alignment property, on the supporting member, and subjecting the alignment treatment, such as a rubbing treatment, to the polymer layer. In the case the rubbing treatment is subjected as the alignment treatment, since the static electricity or the dusts are generated on the surface of the alignment film, a technique, for making the alignment film to exhibit the alignment limiting force without subjecting the rubbing treatment, is studied. The photo-alignment method is one of them, in which the alignment limiting force (anisotropy) is generated on the surface of the alignment film by irradiating light having an optional polarization state (polarized light). Such photo-alignment method includes: the “photo anisotropic type” of reversibly changing the alignment state by changing only the molecule shape; and the “photo reaction type” of changing the molecules themselves. The latter “photo reaction type” can further be classified into the dimerization type, the decomposition type, the coupling type, the decomposition-coupling type, or the like.
In the optical elements comprising the supporting member, the alignment film and the refractive index anisotropic layer as explained above, it is of course important that the optical characteristics of each layer of the optical element are preferable. However, it is also important that the adhesion between the layers and the durability of each layer are preferable. As to the latter problem, for example, if the adhesion between the alignment film and the refractive index anisotropic layer constituting the optical element is poor, there is a problem that the refractive index anisotropic layer can be easily peeled off from the alignment film. Moreover, if the optical element is used or stored in a high temperature and high humidity environment, there is a problem that mesh like wrinkles are generated in the refractive index anisotropic layer.
In order to solve such problems, conventionally, the following techniques have been proposed.
That is, JP-A No. 9-152509 proposes a method in which a modified polyvinyl alcohol is used, as the alignment film material, to improve the adhesion between the layers due to chemical coupling at the interface of the alignment film and the refractive index anisotropic layer. Moreover, JP-A No. 10-10320 proposed a method in which the adhesion between these layers is improved by inserting an anchor coating layer in between layers of a low adhesion. Furthermore, Japanese Patent Application National Publication (Laid-Open) No. 2000-514202 proposes a method in which the durability of the refractive index anisotropic layer is improved by adding a monomer into the material of the refractive index anisotropic layer. Specifically, it proposes a method in which the glass transition temperature, the thermal stability and the mechanical stability is changed by containing a non-mesogen compound having two or three or more polymerizable functional groups by amount of 20% or less with respect to a reactive mesogen compound, as the refractive index anisotropic layer material.
However, among the conventional methods mentioned above, in the method mentioned in JP-A No. 9-152509, since the boiling point of the solvent, used for the modification reaction of the polyvinyl alcohol, is high, the coating solution including such solvent cannot be utilized. Therefore, a refining process of reprecipitation of the polyvinyl alcohol is indispensable, and thus, there is a problem that the manufacturing cost is increased. Moreover, in the method disclosed in JP-A No. 10-10320, when the liquid crystal compound is used as the material for the refractive index anisotropic layer, there is a problem that the liquid crystal molecules are not aligned preferably on the anchor coating layer. Furthermore, in the method mentioned in Japanese Patent Application National Publication (Laid-Open) No. 2000-514202, in the case of providing the refractive index anisotropic layer by fixing the alignment state of the liquid crystal molecules after the alignment treatment, the additives will be the impurities at the time of aligning the liquid crystal molecules so that the alignment of the liquid crystal molecules are inhibited. Thus, there is a problem that the optical characteristics are deteriorated (for example, the display unevenness generation).